The present invention relates generally to lasers for optical communication systems, and more particularly to direct modulation of a selected laser of an array of lasers for an optical communication system.
Semiconductor lasers are used for generating the light that carries data in fiber-optic systems. The most common form of laser for long distance fiber optic communication is the distributed feedback (DFB) laser. In a DFB laser a diffraction grating is embedded in the laser and controls the wavelength of light emitted by the laser. DFB lasers can be made single mode and have narrow linewidths and excellent low noise characteristics, appropriate for fiber optic applications.
Most of the demanding long distance fiber optic links use lasers of different wavelengths, as multiple channels, each at a different wavelength, are transmitted through the fiber simultaneously. A tunable laser that can be configured for any channel is therefore very useful.
A number of configurations for tunable lasers use an array of DFB lasers integrated on the same substrate. Each DFB laser has a different grating pitch and therefore lases, or emits light, at a different wavelength. To select light of a particular wavelength (understanding that the light may include other wavelengths depending on for example linewidth), one laser, or element, of the array is activated and light from the selected laser is coupled to the optical fiber. In many applications, the DFB laser runs continuous wave (CW), that is provides a steady source of light, and the modulation is applied afterwards using a lithium niobate Mach Zehnder modulator or an electro-absorptive device. However, in lower cost applications, it may be desirable to apply the modulation directly to the DFB laser.
Applying modulation to one element of the DFB laser array with an electronic transistor based radio frequency (RF) switch generally requires a high performance switch as well as an array of bypass capacitors and inductors, making the laser subassembly complex and difficult to use. The insertion loss of the switch at high frequencies also limits performance of systems incorporating the switch.